U.S. patent number 5,224,068 [Application Number 07/638,076] was granted by the patent office on 1993-06-29 for recording method for magneto-optic memory medium.
This patent grant is currently assigned to Sharp Kabushiki Kaisha. Invention is credited to Mitsuo Ishii, Hiroyuki Katayama, Tomoyuki Miyake, Kenji Ohta.
United States Patent |
5,224,068 |
Miyake , et al. |
June 29, 1993 |
Recording method for magneto-optic memory medium
Abstract
A recording method for a magneto-optic memory medium of
exchange-coupled type having a recording layer of a low Curie point
and high coercive force and a reading layer of a high Curie point
and low coercive force, which comprises the steps of: applying a
magnetic field to the magneto-optic memory medium to develop a
predetermined data in the reading layer, and then applying both an
optical beam and a magnetic field to the magneto-optic memory
medium for writing the predetermined data in the recording layer,
and simultaneously verifying the data upon the writing on the basis
of a Kerr effect of the optical beam caused by the reading
layer.
Inventors: |
Miyake; Tomoyuki (Nara,
JP), Ishii; Mitsuo (Yamatokoriyama, JP),
Katayama; Hiroyuki (Nara, JP), Ohta; Kenji
(Kitakatsuragi, JP) |
Assignee: |
Sharp Kabushiki Kaisha (Osaka,
JP)
|
Family
ID: |
11542174 |
Appl.
No.: |
07/638,076 |
Filed: |
January 7, 1991 |
Foreign Application Priority Data
Current U.S.
Class: |
365/122; 360/59;
369/13.02; 428/820.6; G9B/11.016; G9B/11.018; G9B/11.046;
G9B/11.049 |
Current CPC
Class: |
G11B
11/10515 (20130101); G11B 11/10519 (20130101); G11B
11/1058 (20130101); G11B 11/10586 (20130101) |
Current International
Class: |
G11B
11/105 (20060101); G11B 11/00 (20060101); G11C
013/06 (); G11B 005/716 () |
Field of
Search: |
;365/122 ;369/13
;428/694,900 ;357/27 ;360/59,114 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
0291248 |
|
May 1988 |
|
EP |
|
0330394 |
|
Feb 1989 |
|
EP |
|
0318337 |
|
Nov 1989 |
|
EP |
|
63-157340 |
|
Nov 1988 |
|
JP |
|
Other References
"Dual-Film Disk Hikes Magneto-Optic Density" Electronics Jun. 1986,
pp. 24-25..
|
Primary Examiner: Popek; Joseph A.
Attorney, Agent or Firm: Conlin; David C. Neuner; George
W.
Claims
What we claimed is:
1. A recording method for a magneto-optic memory medium of
exchange-coupled type having a recording layer of a low Curie point
and high coercive force and a reading layer of a high Curie point
and low coercive force, which comprises the steps of:
applying a magnetic field to the magneto-optic memory medium to
develop a predetermined data in the reading layer,
and then applying both an optical beam and a magnetic field to the
magneto-optic memory medium for writing the predetermined data in
the recording layer, and simultaneously verifying the data upon
said writing on the basis of a Kerr effect of the optical beam
caused by the reading layer.
2. The recording method of claim 1 in which the recording layer is
a GdTbFe amorphous alloy thin film and the reading layer is a
GdNdFe amorphous alloy thin film.
3. The recording method of claim 2 in which each of the recording
and reading layers has a thickness of 100 to 1000 .ANG..
4. The recording method of claim 2 in which the recording layer has
a thickness of 200 to 600 .ANG. and the reading layer has a
thickness of 150 to 600 .ANG..
5. A recording method for a magneto-optic memory medium of exchange
coupled type having a recording layer of a low Curie point of a
GdTbFe amorphous alloy thin film in which the Gd Tb Fe amorphous
alloy is represented by the following formula:
(wherein p and q satisfy the inequalities 0.1<q, 0.35,
0<p.times.q<0.25, 0<(1-p).times.q, 0.25), and a reading
layer of a high Curie point and low coercive force of a GdNdFe
amorphous alloy thin film wherein the GdNdFe amorphous alloy is
represented by the following formula:
(wherein x and y satisfy the inequalities 0.1<.times.<0.3 and
0<y<0.25), which method comprises the steps of applying a
magnetic field to the magneto-optic memory medium to develop a
predetermined data in the reading layer, and then applying both an
optical beam and a magnetic field to the magneto-optic memory
medium for writing the predetermined data in the recording layer,
and simultaneously verifying the data upon said writing on the
basis of a Kerr effect of the optical beam caused by the reading
layer.
6. A recording method of claim 5, in which said optical beam is
substantially a constant intensity and a method of said writing the
predetermined data in the recording layer is applied by an
inversion of the magnetic field.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a recording method for magneto-optic
memory medium, and more particularly, to a recording method for
magneto-optic memory medium including an overwriting to the
magneto-optic memory medium by the use of a magnetic field
modulating process wherein the writing and verifying of data is
simultaneously performed.
2. Description of the Related Art
A thin film of amorphous rare earth-transition metal alloy, such as
GdCo, TbFe, GdNdFe, GdTbFe and the like (hereinafter abbreviated as
RE-TM film) has been used as a memory medium in a magneto-optic
disc device since it has suitable characteristics for magneto-optic
recording. Particularly known is a magneto-optic memory medium
having an exchange-coupled double-layered structure in which a
rccording layer of a low Curie point and a high coercive force and
a reading layer of a high Curie point and a low coercive force are
laminated to improve a reading efficiency of written data
("Magnetization Process of Exchange-Coupled Ferrimagnetic
Double-Layered Films", Japanese Journal of Applied Physics, Vol.
20, No. 11, November 1981 pp. 2089-2095).
Recording in the exchange-coupled magneto-optic memory medium is
performed by applying an optical beam for heating and a magnetic
field to the memory medium to write a predetermined data in the
writing layer. After writing (and cooling), the data is
automatically transcribed to the reading layer having a high Kerr
effect due to exchange-coupling by magnetization, and stably
retained. Hence, reading can be stably carried out by the use of a
Kerr effect of the reading layer which provides an excellent
reading efficiency.
Rewriting in the magneto-optic memory medium is carried out usually
by the steps (1) erasing of old data, (2) writing of new data, and
(3) verification (confirmation of written data). The art at the
primitive stage turns the disc once for each of the above steps, so
that the disc requires to turn three times every rewriting.
In this regard, the so-called overwriting technique indispensable
for a high speed recording process of information has been
positively studied. A magnetic field modulation process is regarded
as a most readily available and effcctive means among various
configurations of the overwriting technique.
The overwrite technique which allows data to be overwritten
performs the aforesaid steps (1) and (2) simultaneously, thereby
making higher a speed of recording process of information.
The verification step (3) after overwriting in the above
exchange-coupled magneto-optic memory medium is to be carried out
in such a manner that the memory medium is cooled enough to cause
data in the recording layer to be fully transcribed to the reading
layer, then, the data is rcad therefrom to be checked. Hence, one
more turn of the disc is required for the verification step after
the overwriting. From this, it is desired to make further higher
the speed of recording process, while the verification step is
indispensable for ensuring reliability of written data and cannot
practically be omitted.
SUMMARY OF THE INVENTION
An object of the invention is to provide a recording method for a
magneto-optic memory medium to perform the aforesaid three steps of
rewriting simultaneously and realizing a high speed recording
process of information.
According to the present invention, there is provided a recording
method for a magneto-optic memory medium of exchange-coupled type
having a recording layer of a low Curie point and high coercive
force and a reading layer of a high Curie point and low coercive
force, which comprises the steps of:
applying a magnetic field to the magneto-optic memory medium to
develop a predetermined data in the reading layer,
and then applying both an optical beam and a magnetic field to the
magneto-optic memory medium for writing the predetermined data in
the recording layer, and simultaneously verifying the data upon
said writing on the basis of a Kerr effect of the optical beam
caused by the reading layer.
According to the present invention, the coercive force of the
reading layer (called hereunder the first layer) in the
magneto-optic memory medium is lower at around a room temperature
than the force of the external magnetic field for writing and the
coercive force of thc recording layer (called hereunder the second
layer and typically about 200 kOe higher force than the first
layer). Hence, the direction of magnetization in the first layer
quickly becomes the same as that of the external magnetic field
without a rise of temperature by the optical beam when the external
magnetic field for writing which is modulated corresponding to
recording signals is applied. As a result, information (a
predetermined data) is transcribed to the first layer. In this
instance, the first layer exhibits a Kerr rotation angle at a level
readily detected at a recording(writing) temperature, so that the
information transcribed into the first layer is detected on the
basis of thc Kerr effect of an optical beam applied to the memory
medium, specifically to the second layer for the writing operation.
The detected magneto-optic signal is used for verification of the
aforesaid information.
Simultaneously with the verification step, the second layer is
heated by the optical beam to have a temperature near its Curie
temperature, and the same information as the aforesaid is written
when the magnetic field for writing is applied to the second layer,
resulting in that the writing of the information and the
verification are carried out at the same time.
Since the second layer has a higher coercive force, it bccomes a
stable data-retaining layer at a room temperature after the writing
step.
Also, the coercive force of the first layer at around a room
temperature is lower than that of the second layer, so that the
direction of magnetization of the first layer follows and is stably
retained by the second layer with a help of an exchange-coupling
force generated between the first and second layers, thereby making
the first layer (the reading layer) and the second layer (the
recording layer) to be stable record-keeping layers.
According to the recording method for a magneto-optic memory medium
of the present invention, the verification of the recorded
information can be carried out simultaneously with recording and/or
overwriting the information, resulting in that the speed of
recording process of information is made higher more than two times
in comparison with the conventional art.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1 through 11 are presented for illustrating an example of the
present invention.
FIG. 1 is a sectional view showing a magneto-optic memory medium
used in the present invention.
FIG. 2 is a graph showing a temperature dependency of a
magneto-optic characteristic measured from the first layer side of
the recording layers of the magneto-optic memory medium.
FIG. 3 is a graph showing a temperature dependency of a
magneto-optic characteristic measured from the second layer side of
the recording layers of the magneto-optic memory medium.
FIG. 4 is a graph showing a hysteresis characteristic of the
magneto-optic memory medium to an external magnetic field.
FIG. 5 is an explanatory view showing a principle of the
magneto-optic recording.
FIG. 6 is a view showing a waveform of detected signals on the
basis of a Kerr effect of an optical beam upon writing.
FIG. 7 is a view showing the other waveform of detected signals on
the basis of a Kerr effect of an optical beam upon writing.
FIG. 8 is a graph showing a compositional dependency of a
magneto-optic characteristic of a GdNdFe amorphous alloy film.
FIG. 9 is a graph showing a compositional dependency of the other
magneto-optic characteristic of a GdNdFe amorphous alloy film.
FIG. 10 is a graph showing a compositional dependency of a
magneto-optic characteristic of a TbFeCo amorphous alloy film.
FIG. 11 is a graph showing a compositional dependency of a
magneto-optic characteristic of a DyFeCo amorphous alloy film.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The exchange-coupled magneto-optic memory medium used in the
present invention comprises a specific recording layer and a
specific reading layer, the layers being laminated on an
appropriate substrate. It is usually preferable that the reading
layer and the recording layer are formed in this order on a
transparent substrate, having thereon an intervening first
dielectric film made of SiN, AlN, ZnS, SiO.sub.2, SiAlON, AlNGe and
the like, and the recording layer is coated with a second
dielectric film.
The recording layer and the reading layer may employ known various
amorphous rare earth-transition metal alloy thin films which are
applied to an exchange-coupled magneto-optic memory medium. It is
preferable that the reading layer has a Curie temperature of higher
than that of the underlying recording layer and the recording
temperature upon application of the optical beam, has a Kerr
rotation angle of readily detectable level at the writing
temperature, and has a coercive force at around a room temperature
being lower than the magnetic field for writing and the coercive
force of the recording layer.
It is particularly preferable that the recording layer is made of
GdTbFe amorphous alloy thin film and the reading layer of GdNdFe
amorphous alloy thin film.
It is preferable that the recording layer comprises an amorphous
alloy thin film represented by the following formula:
(wherein p and q satisfy the inequalities 0.1<q<0.35,
0<p.times.q<0.25, 0<(1-p).times.q<0.25), and the
reading layer comprises an amorphous alloy thin film represented by
the following formula:
(wherein x and y satisfy the inequalities 0.1<x<0.3 and
0<y<0.25).
These amorphous alloy thin films may be formed by sputtering or
deposition, for example, the sputtering process using a target of
alloys having corresponding compositions or a composite target in
an on-chip type, or a multiple-synchronous deposition process using
a multiple source.
Thickness of the recording and reading layers are 5,000 .ANG. or
less usually and 100 to 1000 .ANG. preferably in consideration of
an extent effected by the exchange-coupling force and a recording
sensitivity.
EXAMPLES
Next, an example of the present invention will be detailed with
referring to FIGS. 1 through 11.
First, an example of structure of a medium 1 serving as a
magneto-optic memory medium of used in the present invention will
be detailed. The medium 1 comprises a glass substrate 2 and a
dielectric layer 3 made of AlN, a reading layer 4 (the first layer)
made of GdNdFe, a recording layer 5 (the second layer) made of
GdTbFe, and an another dielectric layer 6 made of AlN, those being
formed in this order on the substrate 2 to form a double-layered
structure by the reading layer 4 and the recording layer 5. GdNdFe
forming the reading layer 4 is an amorphous alloy having a
component ratio as Gd 19.0 Nd 4.0 Fe 77.0 at % and 200 .ANG. of
thickness. GdTbFe forming the recording layer 5 is an amorphous
alloy having a component ratio as Gd 14.0 Tb 14.0 Fe 72.0 at % and
600 .ANG. of thickness.
Temperature dependency of a coercive force Hc and a Kerr rotation
angle .theta..sub.k which are magneto-optic characteristics of the
medium 1 constructed as above were measured from the side of the
reading layer 4 (GdNdFe) and the result is shown in FIG. 2 wherein
the coercive force H.sub.c is shown by the mark o and the Kerr
rotation angle .theta..sub.k by the mark . FIG. 2 reveals that the
reading layer 4 has a low coercive force H.sub.c and is readily
magnetized and inverted to have uniform magnetization direction in
an entire temperature range when a magnetic field higher than 200
Oe is applied to the reading layer 4. Also, the reading layer 4 has
a high Curie temperature, so that a Kerr rotation angle
.theta..sub.k is large and kept at a value 50% or more of that at
room temperature at about 160.degree. C. In a usual magneto-optic
recording, the memory layer is raised of temperature to about
160.degree. C. by an optical beam for writing, typically a laser
beam. Hence, the reading layer 4 made of GdNdFe is provided with a
Kerr rotation angle .theta..sub.k at a level fully and readily
detectable at a writing temperature (for example, higher than 0.1
deg), thereby providing a detected signal having a sufficient
intensity.
Likewise, temperature dependency of a coercive force Hc and a Kerr
rotation angle .theta.k which are magneto-optic characteristics of
the medium 1 were measured from the side of the recording layer 5
(GdTbFe) and the result is as shown in FIG. 3 wherein the coercive
force Hc is shown by the mark o and the Kerr rotation angle by the
mark as in FIG. 2. FIG. 3 reveals that the recording layer 5 has a
coercive force Hc about 2 kOe at room temperature and is very small
at nearly 160.degree. C. of the writing temperature. This is due to
the low Curie temperature of the GdTbFe. Hence, the recording layer
5 made of GdTbFe is readily magnetized and inverted to follow the
direction of the recording magnetic field when the recording
magnetic field is applied to the recording layer 5 at the writing
temperature, resulting in that the recorded information can be
stably retained at around a room temperature.
Next, explanation will be given on that the double-layered
structure enables magnetization direction of the reading layer 4 to
follow that of the recording layer 5 with a help of the
exchange-coupling action. FIG. 4 shows hysteresis characteristic of
a Kerr rotation angle .theta..sub.k measured from the reading layer
side with respect to an external magnetic field H. For measurement,
the reading layer 4 and the recording layer 5 were initialized by
+2.0 kOe of an external magnetic field at room temperature and
subjected to a series of external magnetic field as 0.fwdarw.+1.0
kOe.fwdarw.0 Oe.fwdarw.-1.0 kOe.fwdarw.0 kOe at 60.degree. C. of
atmosphere. In FIG. 4, the values of Kerr rotation angle
.theta..sub.k are constant when the values of the external magnetic
field are positive (the same direction as the initialized
magnetization), and are inverted when the value of the external
magnetic field is about -0.2 kOe. When the applied external
magnetic field is further varied from -1.0 kOe to 0 kOe, the value
of Kerr rotation angle would be inverted again at about +0.2 kOe of
the external magnetic field if only the reading layer 4 is provided
without the recording layer 5. But, the value of Kerr rotation
angle is actually inverted again at about 0.2 kOe and returned to
the original value at the state that the hysteresis loop is
closed.
The first inversion of the Kerr rotation angle at about -0.2 kOe
corresponds to the above explanation that the reading layer 4 is
readily magnetized and inverted in magnetization direction in an
entire temperature range due to an applied magnetic field higher
than about 200 Oe. Also, the fact that the values of external
magnetic field H is inverted again at about -0.2 kOe and returned
to the original value in the state of hysteresis loop being closed
results just from that, when the external field H causing the
reading layer 4 to be inverted was weakened, an exchange-coupling
force between the reading layer 4 and the recording layer 5 caused
the reading layer 4 to be inverted again. It is because the
coercive force Hc of the recording layer 5 is larger than +1.0 kOe
at the measuring temperature 60.degree. C. as shown in FIG. 3, so
that the recording layer 5 is not to be magnetized and inverted by
an external magnetic field in the range +1.0 kOe to -1.0 kOe but
keeps its initialized direction of magnetization. Also, the
coercive force Hc of the recording layer 5 is enough to apply an
exchange-coupling force to the reading layer 4.
As aforementioned, the double-layered structure for recording in
the medium 1 allows the exchange-coupling force to be produced in
order to follow the magnetization direction of the reading layer 4
to that of the recording layer 5.
Next, a principle of the recording method for the magneto-optic
memory medium of the present invention will be detailed. When a
magnetic field 10, which runs in the direction shown by the arrow A
in FIG. 5 and is modulated corresponding to the predetermined data,
is applied to the memory layer of the double-layered structure
comprising the reading layer 4 and the recording layer 5, the
reading layer 4 becomes identical in magnetization direction to the
recording magnetic field 10 before a rise of temperature due to
application of a laser beam 7, so that the information (the
predetermined data) is transcribed to the reading layer 4. This is
because a coercive force of the reading layer 4 is smaller than the
level of the magnetic field 10 as aforesaid. Thereafter, when a
modulated magnetic field 10 identical in information to the above
and the laser beam 7 (for example, of about 4.0 to 10 mW output)
are applied to the recording layer 5, the layer 5 is raised in
temperature to near the Curie temperature, thereby causing a
portion 9 subjected to the elevated temperature to lower in a
coercive force and be magnetized along the direction of the
magnetic field 10.
By turn, the reading layer 4 has a Kerr rotation angle
.theta..sub.k higher enough to be readily detectable at around the
writing temperature, so that a detected signal of a sufficient
intensity can be obtained from reflection of the laser beam 7 on
the basis of its Kerr rotation angle. Since the recorded
information is previously transcribed to the reading layer 4 in the
same direction as that of the magnetic field, the signals detected
from the transcribed information can be applied to verification of
recording information.
The detected signals (output by the use of pick-up) in an actual
overwriting by use of a floating magnetic head is shown hereunder.
The size of the slider of the magnetic head used in the experiment
is 6.times.4 mm to allow the slider to flow about 5 .mu.m above the
surface of medium 1. The magnetic head is of 0.3.times.0.2 square
mm and 1 mm length in a single magnetic pole type with 12 turns of
50 .mu.m .phi. Cu wire, and driving current is .+-.0.4 A. The
magnetic field generated by the magnetic head was .+-.200 Oe.
The detected signals upon overwriting by use of the memory medium 1
of the present invention is shown in FIG. 6. The film thickness of
the dielectric layer 3 is 800 .ANG. and those of the reading layer
4 and the recording layer 5 are 200 and 600 .ANG. respectively as
referred to on the explanation of FIG. 1, and that of the
dielectric layer 6 is 250 .ANG.. The medium 1 before being
overwritten has been written in by use of recording signals of a
single frequency of 1.85 MHz. The tester is adapted to jump one
track after writing in every one track. The recording signal used
for overwriting is of a single frequency of 1.0 MHz.
In view of FIG. 6, an overwriting detection signal 13 shown by the
thick and solid line and having a large amplitude is obtained upon
application of the laser beam of 4.0 mW of recording output, and
its frequency corresponds to that (1.0 MHz) of the recording
signals. Hence, it was confirmed to be possible to verify the
written data upon its overwriting. For the reference, the data
previously recorded upon the frequency of 1.85 MHz was reproduced
before overwriting by the use of a laser beam of 4.0 mW, the result
of which is shown as a detecting signal 14 in FIG. 6.
The detected signals upon overwriting by use of a conventional
magneto-optic memory medium is shown in FIG. 7 as a comparative
example. A magneto-optic memory medium used in this comparison
comprises a four-layered structure including a dielectric layer
made of AlN, a recording layer made of GdTbFe, a dielectric layer
made of AlN and a reflective layer made of Al, those being
laminated in this order on a glass substrate. Also in the
comparative example, the medium before being overwritten has been
written in by recording signals of 1.85 MHz of a single frequency,
and recording signals used for overwriting has 1.0 MHz of a single
frequency.
In view of FIG. 7, since the Kerr rotation angle .theta..sub.k of
the above memory layer is small, the overwriting detection signal
15 shown by the thick and solid line and having a rather small
amplitude is obtained upon application of the laser beam of 4.0 mW
of recording output, and its frequency corresponds to that (1.85
MHz) of a previous recording signal. Hence, it was confirmed that a
previously recorded data was detected, which is of no use for the
verification. For the reference, the data previously recorded upon
the frequency of 1.85 MHz was reproduced by the use of a laser beam
of 4.0 mW, the result of which is also shown as a detecting signal
16 in FIG. 7.
The recording method for a magneto-optic memory medium of the
present invention is not limited in application to the
magneto-optic memory medium having the abovesaid construction.
Various examples of magneto-optic memory mediums applicable to the
present invention will be detailed hereunder.
A composition of GdNdFe forming the reading layer 4 will be
referred to. A compositional dependency of a coercive force Hc of
GdNdFe at room temperature is shown in FIG. 8, wherein a
characteristic when a composition ratio of Nd is fixed at about 4%
is shown by the mark O and that with a Nd composition ratio at
about 10% is shown by the mark . In the case where the Nd
composition ratio is fixed at about 4%, the coercive force of
GdNdFe at room temperature is about 0.4 to 0.9 kOe, but it is
restrained to be less than 0.1 kOe at high temperature more than
100.degree. C. By use of a two-layered structure comprising the
reading layer 4 and a recording layer 5 made of GdTbFe, the values
of a coercive force H.sub.c at room temperature was about 0.15 kOe
within the range of Gd composition ratio about 17 to 25% as seen in
FIG. 2. Also, the Curie temperature of GdNdFe was higher than
190.degree. C. as a whole to fully exceed the writing
temperature.
A compositional dependency of a Kerr rotation angle of GdNdFe is
shown in FIG. 9, wherein a characteristic when a composition ratio
of Nd is fixed at about 4% is shown by the mark o, that with a Nd
composition ratio at about 10% by the mark , and that with a Nd
composition ratio at about 21% by the mark .DELTA.. In the case
where the Nd composition ratio is fixed at about 4%, the Kerr
rotation angle .theta..sub.k exhibits a high value as about 0.4
(deg) within the range of Gd composition ratio about 17 to 25%.
As the result, when a composition ratio of Nd in the GdNdFe forming
the reading layer 4 is fixed at about 4%, a composition ratio of Gd
can be set to be about 17 to 25%.
Any combinations of the reading layer 4 of film thickness in the
range 150 to 600 .ANG. with the recording layer 5 of film thickness
in the range 200 to 600 .ANG. exhibit a substantially identical
characteristic to that disclosed in the present example of the
invention, thereby enabling the overwriting and the verification to
be performed simultaneously.
Also, when the characteristics of GdNdFe of the reading layer 4,
such as a coercive force H.sub.c, a Kerr rotation angle
.theta..sub.k and a Curie temperature are controlled in an optimum
range, the magneto-optic memory medium for the recording layer 5
may be made of TbFeCo, DyFeCo and GdTbFeCo and the like.
Furthermore, such a magneto-optic memory medium may be employed,
which comprises a glass substrate 2 and a dielectric layer 3 which
is made of AlN and has film thickness 800 .ANG., a reading layer 4
which is made of TbFeCo and has film thickness 200 .ANG., a
recording layer 5 which is made of TbFeCo in a different
composition ratio to that of the reading layer and has film
thickness 600 .ANG., and a dielectric layer 6 which is made of AlN
and has film thickness 250 .ANG., those formed in this order on the
glass substrate 2. In this case, a composition ratio of TbFeCo
forming the reading layer 4 was set as Tb 10.0, Fe 82.0, Co 8.0 at
%, and that of TbFeCo forming the recording layer 5 was set as Tb
24.0, Fe 68.0, Co 8.0 at %. A compositional dependency of a
coercive force H.sub.c and a Curie temperature T.sub.c of TbFeCo
when a composition ratio of Co is fixed at 8.0% is shown in FIG.
10, wherein the compositional dependency of a coercive force
H.sub.c is shown by the mark o, and that of a Curie temperature
T.sub.c by the mark . In view of FIG. 10, a coercive force H.sub.c
of the reading layer 4 with Tb composition ratio being 10.0% is
about 0.4 kOe. Also, the recording layer 5 with Tb composition
ratio 24.0% is substantially compensated in composition. It was
confirmed that a magneto-optic memory medium constructed as above
can simultaneously permit the overwriting and the verification.
A compositional dependency of a coercive force H.sub.c and a Curie
temperature T.sub.c of DyFeCo when a composition ratio of Co is
fixed at 19.0% is shown in FIG. 11, wherein the compositional
dependency of a coercive force H.sub.c is shown by the mark o, and
that of a Curie temperature T.sub.c by the mark similarly to FIG.
10. In view of FIG. 11, a composition ratio of DyFeCo forming the
reading layer 4 and that of DyFeCo forming the recording layer 5
can be selected with an optimum value similarly to the above
TbFeCo.
Although the reading layer 4 and the recording layer 5 may be made
of the same material having different compositions, the recording
layer 5 may be made of GdTbFe, GdTbFeCo, NdDyFeCo and GdDyFe and
the like.
* * * * *